The present invention relates to compositions and methods of inhibiting tumors and, more particularly, of treating a malignant tumor, like a melanoma, using plant-derived compounds and derivatives thereof, and in particular, prodrugs of betulinic acid and betulinic acid derivatives.
Over the past four decades the incidence of melanoma has been increasing at a higher rate than any other type of cancer. It is now theorized that one in ninety American Caucasians will develop malignant melanoma in their lifetime. While an increasing proportion of melanomas are diagnosed sufficiently early to respond to surgical treatment and achieve a greater than 90% ten-year survival rate, it is estimated that nearly 7,000 individuals suffering from metastatic melanoma will die in the United States each year.
For patients afflicted with a metastatic melanoma not amenable to surgical extirpation, treatment options are limited. 5-(3,3-Dimethyl-1-triazenyl)-1-H-imidazole-4-carboxamide (dacarbazine, DTIC) is the most efficacious single chemotherapeutic agent for melanoma having an overall response rate of 24%. But the duration of response to DTIC is generally quite poor. Combination therapy with other synthetic and recombinant agents, including N,Nxe2x80x2-bis(2-chloroethyl)-N-nitrosurea (carmustine, BCNU), cisplatin, tamoxifen, interferon-alpha (INF-xcex1), and interleukin-2 (IL-2), has a higher response rate (e.g., 30-50%) in some trials, but a durable complete response rate is uncommon and toxicity is increased. Sequential chemotherapy also has promise, but current treatment options for individuals suffering from metastatic melanoma are unsatisfactory.
Various drugs derived from natural products, such as adriamycin (doxorubicin), bleomycin, etoposide, and vincristine, and their derivatives, have been tested for efficacy against melanoma either as single agents or in combination therapy. However, similar to the synthetic and recombinant compounds, these compounds exhibit low response rates, transient complete responses, and high toxicities.
Nonetheless, as demonstrated by known and presently used cancer chemotherapeutic agents, plant-derived natural products are a proven source of effective drugs. Two such useful natural product drugs are paclitaxel (taxol) and camptothecin. Paclitaxel, originally derived from the bark of the Pacific yew tree Taxus brevifolia Nutt. (Taxaceae), currently is used for the treatment of refractory or residual ovarian cancer. More recently, clinical trials have investigated the possible role of paclitaxel in the treatment of metastatic melanoma. As a single agent, taxol displays activity comparable to cisplatin and IL-2. Taxol functions by a unique mode of action, and promotes the polymerization of tubulin. Thus, the antitumor response mediated by taxol is due to its antimitotic activity.
The second drug of prominence, camptothecin, was isolated from the stem bark of a Chinese tree, Camptotheca acuminata Decaisne (Nyssaceae). Camptothecin also functions by a novel mechanism of action, i.e., the inhibition of topoisomerase I. Phase II trials of a water-soluble camptothecin prodrug analog, irinotican (CPT-1l), have been completed in Japan against a variety of tumors with response rates ranging from 0% (lymphoma) to 50% (small cell lung). Topotecan, another water-soluble camptothecin analog, currently is undergoing Phase II clinical trials in the United States.
In addition, studies have shown that betulinic acid, and betulinic acid derivatives, can inhibit other types of cancer cells, such as neuroblastoma, in addition to melanoma. For example, Das Gupta et al. U.S. Pat. No. 5,658,947 discloses that betulinic acid is useful for the selective control or treatment of human melanoma, and Pezzuto et al. U.S. Pat. No. 5,962,527 discloses the selective activity of derivatives of betulinic acid against melanoma cells.
However, a disadvantage associated with the use of betulinic acid or a betulinic acid derivative in the treatment of a cancer is the problem encountered in formulating these active drugs and in providing suitable dosage forms for the treatment of various cancers. The present application is directed to overcoming this disadvantage and providing useful prodrugs of betulinic acid and derivatives thereof that are easy to formulate into a variety of dosage forms and that release betulinic acid or the derivative thereof in vivo.
The present invention is directed to a method and composition for preventing or inhibiting tumor growth. The active compound is a prodrug of betulinic acid or a betulinic acid derivative which generates betulinic acid or a derivative thereof in vivo. Betulinic acid is a natural product obtained by a method comprising the steps of preparing an extract from the stem bark of Ziziphus mauritiana and isolating the betulinic acid. Alternatively, betulin can be isolated from the extract, and betulinic acid then is prepared from betulin by a series of synthetic steps.
Betulinic acid can be isolated from the extract by mediating a selective cytotoxic profile against human melanoma in a subject panel of human cancer cell lines, conducting a bioassay-directed fractionation based on the profile of biological activity using cultured human melanoma cells (MEL-2) as the monitor, and obtaining betulinic acid therefrom as the active compound. The resulting betulinic acid can be used to prevent or inhibit tumor growth, or can be converted to a derivative to prevent or inhibit tumor growth.
The physiochemical properties of betulinic acid, e.g., a high melting point and limited solubility in hydrophilic and hydrophobic solvents, make it difficult to produce betulinic acid-containing pharmacological formulations. The present invention is directed to providing betulinic acid or a betulinic acid derivative in a form that is easy to formulate and wherein endogenous enzymes can release the active betulinic acid or derivative in vivo.
Therefore, an important aspect of the present invention is to provide a method and composition for preventing or inhibiting tumor growth and, particularly, for preventing or inhibiting the growth of a malignant tumor using a natural product-derived compound, or a derivative thereof, in an easy-to-formulate form.
Another aspect of the present invention is to improve the bioavailability of betulinic acid and betulinic acid derivatives in an individual by administering a therapeutically effective amount of a prodrug of betulinic acid or betulinic acid derivative to an individual in need thereof.
Another aspect of the present invention is to provide a treatment method utilizing a prodrug of betulinic acid or derivative thereof to prevent the growth or spread of cancer cells, wherein betulinic acid or a derivative thereof is administered to an individual in need thereof in a manner consistent with the treatment of a cancer sensitive to betulinic acid or a derivative thereof, e.g., in a topical preparation for the prevention, inhibition, or treatment of melanoma, or intravenously or intraperitoneally for other forms of cancer.
Yet another aspect of the present invention is to overcome the problem of high mammalian toxicity associated with synthetic anticancer agents by using a natural product-derived compound, e.g., a prodrug of betulinic acid or a betulinic acid derivative.
Yet another aspect of the present invention is to provide a composition and method of treating various forms of cancer with a naturally occurring product, or a derivative thereof. In particular, the present invention is directed to inhibiting malignant tumor growth associated with melanoma, neuroblastoma, breast cancer, lung cancer, fibrosarcoma, colon cancer, oral epidermoid carcinoma, epidermoid carcinoma, prostate cancer, hormone-dependent breast cancer, and glioma.
In particular, an aspect of the prevent invention is to provide a composition for treating tumor growth comprising: 
and (b) an optional carrier.
Another aspect of the present invention is to provide a composition for treating tumor growth comprising: 
wherein R1 and R3, independently, are selected from the group consisting of hydrogen, CO(C1-C6alkyl)NR4R5, CO(C1-3alkyl) CO2R4, COCH(C6H5)NR4R5, CO(C1-C6alkyl), CO(C1-C6alkyl) CO2R4, CO(C1-6alkyl)Oxe2x80x94(CH2CH2O)nC1-3alkyl, CH2OCO2C1-6alkyl, CH2OCOC1-6alkyl, PO(OH)2, and SO3H,
R2 is selected from the group consisting of hydrogen, C1-C6alkyl, CH2C6H5, C1-C6alkylNR4R5, CH2OCOC1-C6alkyl, PO(OH)2, SO3H, CH (C6H5)NR4R5, (C1-C6alkyl)CO2R4, and (C1-C6alkyl)O(CH2CH2O)nC1-3alkyl,
R4 and R5, independently, are selected from the group consisting of hydrogen, C1-C6alkyl, CO(C1-C6alkyl), and aryl, or R4 and R5 can be taken together to form a 5 to 7 membered ring,
and n is 1 to 10;
and pharmaceutically acceptable salts thereof,
and (b) an optional carrier.
Yet another aspect of the present invention is to provide a method of treating cancer sensitive to betulinic acid or a derivative thereof comprising administering to an individual in need thereof a therapeutically effective amount of a prodrug of betulinic acid or a derivative of betulinic acid. In particular, wherein the cancer is selected from the group consisting of a melanoma, a squamous tumor, a breast cancer, a colon cancer, a sarcoma, a human oral epidermal carcinoma, a hormone-dependent breast cancer, a prostate cancer, a lung cancer, a glioma, a melanoma, and a neuroblastoma.
These and other aspects of the present invention will become apparent from the following description of the invention, which are intended to limit neither the spirit nor scope of the invention but are only offered as illustrations of the preferred embodiments of the invention.